The present invention relates to a method of soldering electronic equipment by a lead-free solder material and a soldered joint formed by the same.
Up until now, broad use has been made of lead-tin (Pbxe2x80x94Sn) solder alloys for soldering various types of electrical and electronic equipment from the viewpoint of their low melting points and good wettability even in oxidizing atmospheres.
Pb has toxicity, so various restrictions are placed on the handling of Pb and alloys and other materials containing Pb.
Further, the recent growing interest in protecting the environment has been accompanied by tougher regulations on disposal of electronic equipment and other waste using Pb-containing alloys.
In the past, scrap electronic equipment using large amounts of Pb-containing solder alloy was generally mainly disposed by burial in the same way as ordinary industrial waste or general waste.
If scrap electronic equipment using large amounts of Pb-containing solder continues to be disposed of by burial as at the present, the elution of the Pb is liable to have a detrimental effect on the environment and living organisms.
In the near future, disposal of scrap electronic equipment using large amounts of Pb-containing solder alloy only after reclamation of the Pb will probably become mandatory.
Up until now, however, no technique has been established for removing Pb efficiently and effectively from scrap electronic equipment etc. Further, the cost of reclamation of Pb is liable to cause a rise in the cost of the products.
Therefore, there is strong interest in development of a soldering technique using a lead-free solder material.
Some lead-free solder materials have been commercialized such as alloys of Sn with Sb (antimony), Ag (silver), Ge (germanium), Ti (titanium), etc. added complexly, but these are limited to special applications. This is because they do not have the features required in general applications in which used conventional Pbxe2x80x94Sn solder alloys have been used, that is, the low melting point and good wettability, reflowability, and the freedom from reaction with the base material to form a brittle compound layer or embrittled layer.
Disclosure of the Invention
The present invention has as its object the provision of a soldering method and soldered joint able to ensure a strength of joint comparable to that of soldering using a conventional Pbxe2x80x94Sn solder alloy without having a detrimental effect on the environment and without a rise in the cost.
The object can be achieved by the a soldering method characterized by comprising the following steps of:
covering Cu electrodes of electronic equipment by a rust-proofing coating consisting of an organic compound including N and
forming soldered joints on the covered Cu electrodes, by using a solder material consisting of at least 2.0 wt % to less than 3 wt % of Ag, 0.5 to 0.8 wt % of Cu, and a balance of Sn and unavoidable impurities.
The solder material used in the present invention may further contain not more than 3 wt % in total of at least one element selected from the group consisting of Sb, In, Au, Zn, Bi, and Al.
One of the typical applications of the present invention is a printed circuit board of an electronic device. By covering the Cu electrodes to be soldered by a rust-proofing coating comprised of an organic compound including N (nitrogen), long term storability and solder wettability are ensured.
In the past, the practice had been to nickel plate the Cu electrodes, then gold plate them, but this had the defect of a high cost and further a complicated plating process and therefore a long manufacturing time. Further, there was the danger of environmental pollution by the disposal of the waste liquor of the plating.
In the present invention, by the use of the above rust-proofing coating, the cost is reduced and the production time is shortened.
In the past, for rust-proofing the Cu electrodes, a resin coating of rosin (natural pine resin), resin (synthetic resin), etc. had been formed. Since the coating was a thick one of over 20 xcexcm, however, probing during electrical tests became difficult. Due to this and other reasons, cleaning was necessary after the soldering.
On the other hand, the practice has been to use a water-soluble rust-proofing agent to reduce the thickness of the coating so as to eliminate the cleaning after the soldering. That is, the Cu electrodes have been cleaned by etching by a copper sulfate solution etc., then immersed in a solution containing 1000 to 5000 ppm of a water-soluble rust-proofing agent to form a coordinate bond coating.
In the present invention, an extremely thin rust-proofing coating is formed by coordinate bonds (chelate bonds) by the N in the organic compound containing N and the metal. The thickness of the coating is believed to be less than 3000 xc3x85.
As the N-containing organic compound comprising the rust-proofing coating of the present invention, use is made of cyclic compounds of the structural formulas shown in FIG. 1 such as imidazole, benzoimidazole, alkylimidazole, benzotriazole, mercaptobenzothiazole, pyrrole, thiazole, etc.
The characteristics required as a solder material are as follows:
(1) A high wettability with the base material.
(2) The ability of soldering at a sufficiently low temperature so as not to cause heat damage to the electronic equipment being soldered. That is, a melting point equal to the melting point of conventional Pbxe2x80x94Sn solder of 456 K (183xc2x0 C.).
(3) Freedom from reaction with the base material to form a brittle intermetallic compound or embrittled layer.
(4) The ability to be supplied in a form enabling application to automation such as a paste, powder, or thread solder.
(5) Freedom from poor wettability, voids, bridges, and other defects due to oxides of the metal ingredients in the solder material.
In particular, in soldering electrical equipment, the molten solder has to be made to flow into narrow clearances, so the surface tension, viscosity, fluidity, etc. of the solder material are important.
The conventional Pbxe2x80x94Sn solder alloys satisfy the above conditions well, but it has been difficult to avoid environmental pollution due to Pb.
The solder material used in the present invention is an Agxe2x80x94Cuxe2x80x94Sn alloy consisting of at least 2.0 wt % and less than 3.0 wt % of Ag, 0.5 to 0.8 wt % of Cu, and the balance of substantially Sn. Since it does not contain Pb and since the alloy ingredients Ag, Cu, and Sn are all elements with a high safety, there is no fear of environmental pollution. Further, the above required characteristics are sufficiently satisfied.
The reasons for limiting the composition of the Agxe2x80x94Cuxe2x80x94Sn solder alloy of the present invention are explained below.
[Ag: at least 2.0 wt % and less than 3.0 wt %]
Regarding the most basic characteristic of a solder material, its melting point, it is necessary to secure a low melting point (not more than 220xc2x0 C.) equal to that of the conventional Pbxe2x80x94Sn solder alloys. If the Ag content is at least 2.0 wt %, a low melting point of not more than 220xc2x0 C. can be secured. If the Ag content becomes less than 2.0 wt %, the melting point suddenly rises. On the other hand, if the Ag content becomes 3.0 wt % or more, a large amount of needle crystals are produced, the electronic devices short-circuit with each other, and the reliability of the joint falls. For applications where it is particularly necessary to prevent short-circuits due to needle crystals and watch the reliability of the joint, it is preferable to further limit the Ag content in the range of the present invention to not more than 2.5 wt % since the production of needle crystals can be substantially completely prevented. Conversely, for applications where it is necessary to particularly keep down the thickness of the intermetallic compound layer explained later, it is preferable to further limit the Ag content within the range of the present invention to not less than 2.5 wt % since the thickness of the intermetallic compound layer can be further reduced. As the Ag content for simultaneously satisfying both these conditions, 2.5 wt % is most preferable.
[Cu: 0.5 to 0.8 wt %]
The solder alloy and Cu electrodes are joined by the production of an intermetallic compound at the interface between the solder alloy and the Cu electrodes. That is, the production of an intermetallic compound is essential. On the other hand, if the intermetallic compound layer becomes too thick, it becomes brittle and the strength of joint falls. Therefore, the intermetallic compound layer is preferably formed as thin as possible at the time of joining. It is preferable that it be resistant to growth due to the heat history after joining. The inventors measured the thickness of the intermetallic compound layer at the joint interface at temperatures of up to 150xc2x0 C., believed to be the heat history to which electronic equipment is subjected in the usage environment. As a result, they discovered that when the Ag content is in the range of the present invention and the Cu content is in the range of 0.5 to 0.8 wt %, the thickness of the intermetallic compound layer stabilizes and can be suppressed to not more than about 4 xcexcm. Even if the Cu content is only a little over the above range, the thickness of the intermetallic compound layer increases. As a Cu content able to suppress the thickness of the intermetallic compound layer to the thinnest, 0.7 wt % is most preferable.
Due to the above reasons, in the Agxe2x80x94Cuxe2x80x94Sn solder alloy of the present invention, the Ag content is limited to at least 2.0 wt % and less than 3.0 wt % and the Cu content is limited to 0.5 to 0.8 wt %. The Ag content may in accordance with need be selected to be in either the range of at least 2.0 wt % to not more than 2.5 wt % or at least 2.5 wt % to less than 3.0 wt %. The most preferable composition is 2.5 Ag-0.7 Cxe2x80x94Sn.
In general, if the soldering temperature of electronic equipment falls by 10 K (10xc2x0 C.), the lifetime of electronic devices is said double. Reduction of the melting point of the solder material is extremely important.
Further, the Agxe2x80x94Cuxe2x80x94Sn solder alloy of the present invention has properties extremely close to the main ingredient Sn, is good in wettability with Cu, and has a high conductivity.
Further, since the amount of Ag added is small, the alloy is provided inexpensively at the same level as a conventional Pbxe2x80x94Sn alloy.
The solder alloy of the present invention can include a total of not more than 3 wt % of one or more elements selected from Sb (antimony), In (indium), Au (gold), Zn (zinc), Bi (bismuth), and Al (aluminum) in addition to the above basic composition of Agxe2x80x94Cuxe2x80x94Sn.
These elements (in particular In and Bi) further lower the melting point of the solder alloy and further improve the wettability. If the total amount is over 3 wt %, however, the appearance of the soldered joint, in particular the luster, is degraded. Further, if the content of Bi alone is over 3 wt %, the reliability of the joint with a Pb-containing material falls.
The solder alloy of the present invention contains as unavoidable impurities O (oxygen), N, H (hydrogen), etc. In particular, O is liable to make the alloy brittle, so preferably should be kept very small in amount.
A solder alloy comprised mainly of Sn is susceptible to oxidation of the Sn at the time of soldering. Therefore, the soldering is preferably performed in an N2 or Ar (argon) or other nonoxidizing atmosphere. Due to this, it is possible to prevent poor wetting or poor electrical connection due to oxidation of the solder alloy.
The soldering of the present invention, like the conventional soldering, can be performed while applying an ultrasonic wave so as to promote wetting.